Originally Posted by TokyoSeven
Mechbox.com has clear concise step by step videos as to how to take about your M4 open up its gear box and install a new spring.
The only upgrades I would recommend for the new spring would be a new spring guide (systema, modify or whatever meets your budget) some new bushings and a reshim and a regrease once its all installed.
All the details are available on mechbox.com.
Just out of curiosity though, are you sure its a systema m110? I wasnt aware systema had a 110 series for AEGs. Anyways best of luck, in case you are interested here is a link to ILLusions spring guide he created a while back.
Keep in mind though that a stronger spring cause more stress on a mechbox and may weaken its durability overtime, how much time its hard to say but Im sure you will be fine for atleast a while.
Yes...Systema has a M100, M110, M120 & M120S and Velocity Arms are selling them for $12.95 each right now. Here's the LINK
Thanks for the mechbox.com site...it's an awesome site/guide!!!